1. Field of Invention
The present invention relates to a process for the making of molecular-specific optical fiber probe coatings for Raman spectroscopy, specifically SERS qualitative and quantitative detection of molecular or ionic species.
2. Background
Contaminant analysis of water is an important requirement and many current methods require the taking and removal of samples. This may inherently degrade the sample resulting in spurious results. For instance, in sampling groundwater exposure to air, contaminating collection procedures, or dirty sample containers may degrade the results. An in-situ, or direct, procedure is needed. Early attempts centered on fluorescence and absorption spectroscopy; however, SERS using fiber-optic probes has a great potential to satisfy this requirement.
Raman spectroscopy, particularly when surfaced enhanced which usually is referred to as SERS, is a proven and effective technique for qualitatively or quantitatively detecting specific chemical compounds within liquids. In Raman detection a probe light created by a laser excites molecules to higher energy states often in their vibrational energy bands. In returning to equilibrium, emission of Raman signal photons occur which characterize the molecular species. This emitted light can then be analyzed to determine the specie and its amount in the solution.
Surface enhancement of the Raman signals makes them orders of magnitude more intense and thus easier to detect; therefore, much lower concentrations can be resolved. Said enhancement is commonly achieved by adsorbed surface compounds containing silver, gold or copper. In many applications of SERS, optical fibers become the medium for transmitting light signals; thus, processing of said optical fibers may improve the SERS response.
Conventional Raman spectroscopy is well covered by D. A. Long's excellent treatise Raman Spectroscopy, McGraw-Hill 1977. A good review of SERS can be found in Keith T. Carron's dissertation, Surface Enhanced Resonance Raman, Resonance Hyper-Raman, and Hyper-Raman Spectroscopy of Molecules Absorbed to Thin Metal Films, Northwestern University 1985; Chapter 2, "Experimental Techniques for Thin Film SERRS", pages 13-45 is herein incorporated by reference, and hereinafter referred to as Carron (1985).
For the subject invention Raman spectroscopy is applied by calibration of the frequency shifts of the returning photon beam. These spectral peaks represent a specific molecular vibration which can be related to the peak height and correlates to the concentration of analyte. However, certain interferences occur between molecular species that mask the individual response peaks and make quantitative determinations especially uncertain. Further developments in Raman spectroscopy have tried to overcome this situation.
Recent U.S. patents covering the above mentioned concepts include;
______________________________________ U.S. Pat. No. Inventor Year ______________________________________ 4,395,312 McCreery et al 1983 4,573,761 McLachlan et al 1986 4,674,878 V0-Dinh-1 1987 4,781,458 Angel-1 et al 1988 4,804,930 Simic-Glavaski 1989 4,834,497 Angel-2 1989 4,999,810 Vo-Dinh-2 1991 ______________________________________
Referring to the above list, McCreery discloses electrochemically generated chromophores in solution that can be detected by Raman probes. McLachlan discloses a fiber-optic probe comprising separate transmitting and collecting optical fibers useful for Raman spectroscopy that have a specially designed angle of convergence. Vo-Dinh-1 discloses SERS substrate of cellulosic material coated with roughness-imparting latex-like microbodies and a metallized coating, preferably of silver, with a thickness of between about 100 to 2000 angstroms. Angel-1 discloses an optical fiber probe, or optrode, thinly coated with SERS metal at a thickness of about 10 to 50 angstroms, and additionally side-only coated over the metal with selectively absorbent material. Simic-Glavaski discloses a molecular electro-optical transistor and switch that can supplement Raman spectroscopy with electrical ionization designed to represent logical or switchable information. Angel-2 discloses a fiber-optic fluid detector designed for a specific material, such as a toxic fluid like gasoline. Vo-Dinh-2 discloses a data storage system using the principle of an SERS substrate as a means of accepting optical information.
The subject invention involves a method of producing optical fibers or fiber-optic probes coated with an appropriate chemical layer that selectively complexes or partitions desired molecules allowing SERS detection.